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redwolfoz writes "New Scientist is reporting that the speed of gravity has been measured for the first time. 'The landmark experiment shows that it travels at the speed of light, meaning that Einstein's general theory of relativity has passed another test with flying colours.' Researchers made the measurement of the fundamental physical constant with the help of the planet Jupiter. One important consequence of the result is that it will help constrain the number of possible dimensions in the Universe."

Yeah, that's the real trick. For those who aren't aware, getting gravity to "play nice" with both general relativity and quantum mechanics is pretty tough. Relativity models gravity is a warping of space. But coming up with a quantum theory of gravity is mighty difficult. There are theories that gravity acts through particles (the so-called gravitons you always hear about on ST:TNG) but I don't believe this has been proven yet.

Einstein's general relativity actually predicts the existence of gravity waves and gravitons (really the same thing, viewed two different ways). Trying to find gravity waves is one of the biggest scientific challenges of our time.

It's accomplished via huge (4 ft. diameter, 2.5 mi. length) tubes in an L-shape. A laser is then bounced along the length of the tube, and measures its distance very accurately: to within 10^-16 (!) cm, or about one hundred millionth the diameter of a hydrogen atom. Any change in the distance is a possible indication of a gravity wave passing through from some distant, powerful source. The fact that gravity decreases exponentially with distance means that even gravitational waves from extremely powerful sources, like binary neutron-star systems, are very weak when they get to Earth.

Of course, other vibrations can screw this up, so these observatories are really isolated (both geographically and mechanically) and data is compared from around the world. Lots of information is available at the LIGO [caltech.edu] (Laser Interferometer Gravitational-Wave Observatory) website, where I got most of the specs listed here.

The fact that gravity decreases exponentially with distance means that even gravitational waves from extremely powerful sources, like binary neutron-star systems, are very weak when they get to Earth.

Gravity is a long-distance force that decreases as inverse distance squared. This is Newton's famous 1/r^2 law, and it remains unaltered by the theory of general relativity (after all, Newton's laws are just a limiting case of General Relativity.)

With a short-range gravitational force, decaying exponentially with distance, stable planetary orbits and galaxies, with their literally astronomical extent, could not exist.

"Remember the Unified Field Theory? Well, forget it. Physicists have pretty much thrown in the towel on unifying gravity with the other elemental forces, so now we have the Standard Model, which says that
everything works together in intricate harmony except gravity, which is on holiday in Tasmania and need not concern us further."
- Jon Carroll on the Higgs Boson

If they have confirmed that gravity travels at the speed of light, how does gravity escape a black hole? obviously it does because the only energy that escapes a black hole is in the form of gravitational waves, but if the escape velocity is higher than than the speed of light, how can it get out?

Stephen Hawking's A Brief History of Time describes it as a series of virtual particles emitted by all matter. They're spin 1 particles (I think - whatever that means). Something about how many times you have to rotate the particle to get back to the same perspective as when you started. Like spin 2 particles, you turn them around 1/2 revolution and you're back to where you started. Spin 1 particles need to be rotated all the way around once. Spin 0 particles look the same in every direction. Spin 1/2 particles (which matter is made up of) have to be rotated twice to get back to the same position. They're called virtual particles because they can't be directly seen.

Anyways, since gravity is made up of particles, it follows the same rules as all other particles, and thus can't go any faster than the speed of light.

I don't think force particles (photons, gravitons, gluons..) have mass, otherwise they wouldn't be able to travel at the speed of light. According to good ol' string theory the vibrations of the strings that make up these particles cancel each other out making them have zero mass. A bit of a leap, and I'm not explaining it very well either.

I haven't read about gravitons but both photons and virtual photons have energy which is mass: E = M (I left out the unit conversion constant "C"; pick different units to measure with and it goes away).

To make that situation slightly clearer, a spinning top is heavier than a stationary one and a charged battery is heavier than a decharged battery.

The rule about virtual particles getting their energy from nowhere is that they don't exist very long. The more energy or mass involved the shorter their life-span. You can make up analogies about this, but that just hides the mystery.

In any case, making gravity a particle doesn't sound like it relates to the curving of space-time that relativity deals with. I wonder how that works!

A photon is always moving at the speed of light relative to any observer. The entirety of its energy is, I believe, contained in the relativistic momentum term of the energy equation... applying e=mc^2 is not relavant here since that equation only applies for objects at rest.

The relativistic energy/mass equivalence equation is (as was reminded to me by another poster):

e^2=(m*c^2)^2 + (p*c)^2

I hope I got the units correct on that. In this equation m is the rest mass (m_0) and this equation is generally equivalent to the E=m_0*gamma*c^2 form, except that form doesn't work too well to describe zero rest mass particles (because you divided out a mass to transform the equation - leaving you with a 0/0 term that is fairly meaningless). Thus all the energy of a photon can be contained in its momentum energy rather than its rest energy.

The actual argument that the rest mass term is zero is based on gauge invariance of QED, and I believe the simpler argument is that a non-zero rest mass of a photon would result in a damping term in Coulomb's law which is not observed (or at least we don't believe it exists, and we've bound it to be nearly zero by our observations).

You kids and all your fancy-schmancy gravity experiments. Back in my day, we showed a little more respect for our elders... and for gravity because it tells you up's up and down's down. Once you start foolin' around measuring this gravity speeds this and that gravitron's that and the next thing you know you'll be flung up into the air and there won't be a down to go to. Bagh. That will serve you right for messing around with things that you should have a little respect for.

I had to walk up hill to school both ways and because of gravity, I liked it!

Observe how it makes a graceful parabola before walloping your cow-orker on the nut.

Now remember I said Mass warps space-Time. Time is just another dimension. We live in a 4 dimensional space.

We measure time in this silly "seconds" unit but thats stupid. We already have a useful unit for spatial dimensions and that is meters. The conversion from meters to seconds is simply the speed of light. 1second=3e8m

So that graceful parabola is actually very stretched out along the time dimension. It is quite a flat curve.

Now retrieve your pen from your irate colleague and hurl it hard at him.

Note that the curvature seems a lot less, but it took less time. Thus you will find in 4 space the curvature was the same.

Now retrieve pen from the poor sods ex-eyeball and lob it very slowly and gently. Note the curvature seems less, but it takes more time. So in 4 space it is the same curvature.

Carefully, carefully retrieve your pen from your ex-friend and use it to calculate the radius of curvature for all three throws. Wow! Its the same!

And it is the radius of the earth.

In other words, Mass warps space time. There is no gravity, only bent space time.

This is exactly the picture introduced by Einstein's general theory of relativity. An object's trajectory is simply the path of least action in warped spacetime. Spacetime is warped because of the presence of masses.

And it is the radius of the earth.

Now I don't know where you got this statement from. First of all, the radius of the earth and the curvature of spacetime near its surface can't be the same, because those two quantities have different units. It would also be very surprising if they turn out to be numerically the same, since the amount by which an object distorts spacetime is not determined by its size, but by its mass, which is size multiplied by density.

There is no gravity, only bent space time.

It seems to have eluded you that Gravity is the name we attribute to the phenomenon that mass bends spacetime. Hence your statement really says: "There is no gravity, only gravity."

Other than that, I really like your illustrative description of how gravity works!

Now if we could only figure out why and how gravity works, we'd be in business.

There is a theory that explains why gravity is directly proportional to mass, and why it's always attractive: that gravity is anti-mass. This is not the same as antimatter, which has the same mass as normal matter but reversed charge.

When matter is created from energy, that energy goes into deforming the rubber-sheet of space-time. The main idea of the theory is that space-time doesn't like being deformed (in precise, conservation law terms), and so creates a restorative energy called gravity. Since matter always has positive mass, the energy of the gravity field must always be negative, i.e. always attractive, and always precisely proportional to the mass.

In effect, when you create matter you are borrowing energy from somewhere to deform space-time. That energy could be kinetic from atoms in atom-smashers, or from energy-to-matter conversion in stars, or any number of other sources.

If you read Einstein, you know that there are some forms of electromagnetism that create gravity (most don't, including plane waves of light). According to general relativity, the EM field is more fundamental than the gravity field, and so in theory it should be possible to create gravity waves using electromagnetism. For more info, see page XX of the preface to The Geometry of Einstein's Unified Field Theory [amazon.com].

Well, actually that's not actually true, depending on how you define weak (the adjective). Gravity exerts the smallest force, but it does so over the greatest distances. OTOH, Electromagnetic forces are very powerful, but only over short distances. The nuclear strong and weak forces fall in the middle accordingly.

If I am not mistaken, nuclear weak force is fairly rare, compared to the others. I always thought nuclear strong was the strongest, electromagnetic 2nd, then weak, then gravity.
"Gravity is not a force, it is a product of space and time"
I love physics, and am considering doing physics in college.

The strong force holds the quarks in the nucleons together, and this also holds the nucleus together - as the nucleons get close to each other, they're able to "see" the quarks inside each other, and are attracted. The strong force is actually infinite range, but appears to be a limited range because quarks are always bound into colourless states, and the strong force works on colour charge.

The weak force mediates between various particle decays etc, the most well known of which is beta decay, where a neutron turns into a proton, electron (and an electron neutrino).

Actually if I'm not mistaken, gravity and electromagnetic both exert a 1/r potential (1/r^2 force). So their distance range is the same.

So at short distances:

strong>weak>=electromagnetic>gravitationa l

but this isn't quite right because the strong force has two characteristics, a main force and a residual force. The main force is what keep quarks together in neutrons and protons. That's absurdly strong, and it's strength actually INCREASES with distance. However, the residual strong force is what keeps the nucleons together, and falls off really fast with distance, like the weak force.

At long distances things are a bit simpler:

Electromagnetic>Gravitational>strong>weak

The problem with this is that the electromagnetic and gravitational are relative. You can't go by the constants associated with the field, because they're defined by us (for example, what if mass were in terms of petagrams? Then G~10^19, and the force (in terms of petanewtons, I think) would skyrocket.)

Point is that it's all dependent upon the system you're talking about and the units you're talking about them in. We really can't compare them.

A point of confusion which seems to appear repeatedly in this thread is that, while the electromagnetic (EM) force seems to be stronger than gravity at microscopic scales,

the inverse square law implies that the ratio of these forces should remain constant with distance, but

everyday experience and astronomical evidence seems to suggest that gravity grows stronger than the EM force at macroscopic scales

I think the key to resolving this conundrum is to realize that the EM attraction is proportional to the relative charge difference between two bodies.

At microscopic scales, one is often dealing with individual EM charges, so the relative charge difference at that scale is large and the force is strong.

In macroscopic objects, it is difficult to separate macroscopic amounts of charge precisely because the EM force is quite strong, so macroscopic objects usually have relatively small charge differences and the macroscopic EM force seems relatively weak.

Compare this to gravity, which only has one type of charge--mass--which always increases as the size of the object increases.

At microscopic distances, you only have small amounts of charge associated with a weak force, so gravity seems weak

With macroscopic objects at macroscopic distances, you have lots and lots of charge associated with a weak force--enough to make gravity appear stronger than EM.

> "The order of universal forces, from strongest to weakest, is Electomagnetic, Strong, Weak, and Gravitational. So gravity, you see, is the weakest force in the universe."> > Try telling Sonny Bono that.

Au contraire! Blunt force trauma is all about electromagnetsm. (I suppose there are a few places where it's also about electroweak interactions, but that's a hell of a lot more trauma than I care to talk about. *g*:)

At any rate, gravitational forces had accelerated Sonny pretty gently, and he was doing just fine until electrostatic forces from a nearby tree intervened.

Sonny was a silly clam (silly clam? I repeat myself) who tried to make the electrons in his body occupy the same space as the electrons in aforementioned tree. (For a guy who claimed to be a great physicist, L. Ron Hubbard sure didn't teach his disciples much about the Pauli Exclusion principle or Van der Waals forces.) Sonny Bono's failure to grasp rudimentary physics can be seen as yet another case of evolution in action.

There is no strong force. It's a myth. Just like Neutrons are a myth. No, I'm not joking. Anytime you extract a neutron from an atom, it breaks into a proton and an electron (hydrogen). A Neutron is not a true particle, it's simply a compressed proton and electron.

First of all, you're still going to have an interesting time explaining how all of those nice, positively-charged protons are bound into an incredibly tiny space without the strong force holding them together.

Secondly, the production and decay of neutrons is mediated by the weak force, not the strong force.

Thirdly, your model fails to explain mesons and the zoo of other particles that can be produced even in relatively low-energy accelerators, while the quark model explains it nicely.

Protons become neutrons when an electron and an "up" quark interact to produce a "down" quark and an electron neutrino. The inverse process - decay of neutrons into protons and electrons - happens when a "down" quark decays into an "up" quark, emitting an electron antineutrino and an electron.

The neutrino emitted during the decay has significant momentum. Its existence can be shown - and was originally inferred - by tracking the charged particles emitted when a neutron decays into a proton and an electron. In many cases, both of the charged particles are going in the same direction. To conserve momentum, something else had to be fired off in the opposite direction during the decay. That "something" is the neutrino. If a neutron was a bound proton/electron pair, there would be no third particle to explain the momentum discrepancy.

You're also overlooking the fact that a bound system has less energy than an unbound one. Which would mean that in your proposed scenario, _neutrons_ should be the stable nucleon, which is at odds with observations.

Or, you may have written that post as sarcasm. Either way, moderators have been falling for it.

The rest of your post is even sillier, so I'm not going to bother with it.

A photon delivers an impulse when it is fired or when it is destroyed on impact with matter - but when it is in transit in space it has no mass.

Imagine a giant cluster of light, like fired by a superlarge pulse laser. It will transfer momentum to whatever it hits, but it does not actually have mass, so when its in transit this massive ball of light will not suck in anything with its gravity.

How about this: a photon has zero rest mass. However, it is never at rest, but travelling at C. It does have energy, which translates to a very little mass and does warp space time, but when it hits something, that energy goes away, and so does the photon.

I wonder if a sufficient density of photons would collapse into a black hole.

It's much more likely the ringing comes from the air right next to the polished gong surface suddenly heating up.

There's a similar confusion about what drives those "solar radiometer" things - you know, a little black-and-white paddlewheel inside an evacuated glass ball that spins when you shine a light on it? People often say the reason they run is photon momentum, when the actual explanation is that the black sides of the paddles are hotter than the white sides, so when the few gas molecules left inside the ball hit the paddles, they leave the black sides going faster than the white sides.

The proof of this is the direction the paddlewheel turns - it turns white-side-first, and a photon-mass explanation would have the paddle turning black-side-first. If you put a paddlewheel inside a REAL hard vacuum, with a REAL low friction bearing, and REALLY isplate it from outside vibration, it turns the right way. See here [ucr.edu] for a more coherent and complete explanation.

There was another poster that also claimed this would have been due to the air heating up near the gong.

However, this professor continued his demonstration with sooting the gong heavily (taking it from polished to near-black), and then firing the flash again. The sound was significantly softer, noticable by all attendees (around 120), and he explained this by the photon package having been absorbed instead of bouncing (the gong only got half the impulse from before).

In a scenario where heat was the cause of the sound, sooting the gong would have caused a significantly louder sound as the light was absorbed, instead of (as happened) as softer one.

In a scenario where heat was the cause of the sound, sooting the gong would have caused a significantly louder sound as the light was absorbed, instead of (as happened) as softer one.

If the gong is reflective, the air near it gets heated both by the incoming light and by the reflected light. If the gong is sooted, only the incoming light heats the air.

At least, this seems logical to me. A way to test it would be to put a vibration sensor on the gong, and try it both in air, and in a vacuum. If you're right, the sensor should read the same, if I'm right the impact in vacuum should be much less.

Yes, but it's the photon's transfer of momentum into compression of deuturium/tritium that makes fusion possible (because merely heating uncompressed deuturium to fusion will not work--this was the fundamental problem of the H-bomb.

Well, except that in the initial design, at least, they used an intermediate stage to transfer the momentum from the radiation pressure (generated by a conventional fission bomb)...they use the radiation to ablate the outer surface of a cylinder of U-238 (natural uranium) surrounding the deuturium/tritium to use the uranium to compress it, which also trips an initiator placed with some U-235 centered in the center of the deuturium, causing it to fission, which creates two massive pressure waves, an incoming and outgoing, that compresses the deuturium mightily. This ignites fusion in it, which, in turn, releases enough fast neutrons to ignite fission in the normally unfissionable U-238 that surrounds it. The fissioning of U-238 actually produces most of the yield of this device, "Mike", which was about a megaton. Quite an intricate piece of work, really.

Now, what did this have to do with this discussion again? Oh, yeah, the momentum of photons. I guess it's marginally ontopic. (Please forgive me, I just finished reading about the development of the H-bomb and couldn't keep from showing off the neato stuff I just learned.)

My high school physics teacher pointed many years ago when we were looking at nuclear energy, that fusion is 'easy'.

Well, he was wrong. It wasn't easy. You can't just somehow heat some nuclear fuel to fusion temperatures and achieve a reaction that will sustain itself. It's possible that the idea of compressing the fuel would have come more quickly if Teller hadn't pounded it into everyone's heads that compression wouldn't make a difference. But it does make a difference in how fast the burning fuel radiates energy away that would otherwise sustain the reaction. Teller started thinking about a fusion bomb as early as 1943, it wasn't until about '49 or so that Ulam had the insight about compression. Of course, his was just a rudimentary idea; Teller quickly proposed using radiation pressure instead and the idea of the "spark plug" at the center quickly followed. There is another way to build a fusion bomb, and it's a layering of fission and fusion in a sphere or something similar. This gets really massive really quick and has an upper yield limit probably less than a megaton. This was how the Russians, independently (which is significant since much of their bomb program was built upon thorough intelligence, not just from Fuchs, about the American and British programs) produced their first h-bomb, about a year or so later, with a yield of about 400kt. But we'd already built pure fission bombs with a greater yield than that, not to mention how we'd already improved our h-bombs very quickly.

Anyway, it's true that just igniting some nuclear fuel into fusion isn't that hugely hard, assuming that you have some tritium, not just deuturium, around. But you don't get that much from it compared to the fission bomb you've exploded to burn that small amount of fuel. In regards to power plants, of course using the heat of the core of a fission explosion is not an option for initiating fusion. And all our current technologies currently use about as much energy to initiate and contain fusion in a fuel than they are to usefully extract from it. The vast gulf seperating fission from fusion power is that once you understand the neutron-capturing cross-sections of various isotopes, cobble together a sufficient mass of an approriate fuel, and find a moderator (and moderator arrangement) to go with it, the actual physical, engineering complexity of the reactor is minimal. You could build one by hand, which is essentially what Fermi did. You can control one by winching a control rod into and out of a pile. In contrast, the fusion reaction is very different in this context and an implementation and control mechaninism is fiendishly complex. I suppose that in a way your teacher was right, in the sense that a fission reactor is very, very different from a bomb; while a fusion reactor must by necessity in some qualitative sense be pretty similar to a fusion bomb.

Photons are not particles in the sense of neutrons, electrons et. al which are massy particles.

Photons are better described as 'packets of energy'. Gravity doesn't just affect mass - it affects energy as well. Light doesnt get 'pulled into' a black hole, it just gets redshifted so much (by the gravity sucking the energy out of it), that its wavelength becomes infinite, and thus immeasureable.

Photons can exert a pressure though because they have MOMENTUM. Thus they have a 'mass equivalent', but they do not have mass, and that is not why they cannot escape black holes.

Light does not have mass. This is why it is capable of travelling at the speed of light-- it is impossible for anything with mass to travel at the speed of light. The reason light can get pulled into a black hole is becuase an object with gravity warps the space around it-- the more mass you have, the more that you warp space. When light gets "pulled into" a black hole, the light is in fact moving in a straight line-- it's just that space is curved in the vicinity of the black hole, so if you travel straight forward you will get pulled into the black hole.

Think of it like you've got a matress, and you have something very heavy sitting on the matress, and the matress is kind of indented in the area of the heavy object. Now imagine if you take a marble, and roll it in a straight line toward the dent the heavy object makes. It will go in a straight line, then when it enters the dent it will start kind of curving around the inside of the dent. The standard metaphor is that gravity works like that, light is like the marble moving in a straight line, but its path is being "bent" by the curvature of space. A "black hole" is an object so heavy it's managed to tear through the fabric of the matress, meaning it's impossible for anything that's fallen into its area to roll back out.

it is impossible for anything with mass to travel at the speed of light.

This is wrong. It is impossible for anything with mass to accelerate to the speed of light. If it is already travelling that speed it can continue indefinitely.

the light is in fact moving in a straight line-- it's just that space is curved in the vicinity of the black hole,

This is semantics. If I throw a softball straight ahead the ball actually moves in a straight line. It is just that space is curved in the vicinity of the earth. Gravity works the same way regardless of the density of the body creating the gravity, so long as the mass is the same.

The same way an electric field doesn't have a charge, but affects objects that do have a charge. Gravitational/electric fields are -created- by masses/charges. And don't confuse gravity with gravity waves (the speed of which being what are measured here).

By the way, did anyone else find the quoted margin of error of.25 to be kinda ridiculous? So based on their measurements, the speed of gravity could actually be anywhere from 30% slower to 20% faster than light. I mean, the article makes it sound like they're just assuming the real number is 1.0 c because anything else would be really surprising. Or maybe the article is wrong. Or I'm mis-reading it. But at the moment, it doesn't sound like "passing with flying colors" to me.

Relativistic speeds are usually measured in terms of gamma, not meters per second. Gamma is a value that represents the amount of time dilation and mass increase an object has; if you're moving at 86% of the speed of light (~206257211 m/s) then gamma is ~2.0, meaning that time would run twice as fast for you, and to a relatively stationary observer, your mass would be double what it is at rest. Gamma is calculated thusly:

y = 1 / sqrt(1 - (v^2 / c^2))

Gamma can rise unbounded; as your velocity approaches light, gamma rises exponentially, reaching infinity when your velocity is equal to that of light. I'm assuming that the original paper used values of gamma for measurement, rather than meters per second.

Almost. If the American GOVERNMENT has anything to do with it. The people and scientists are not on the whole evil and destructive like our government is.

Your definition of evil must be the common "has different priorities or beliefs than I do and isn't perfect"

There are better choices for a definition of evil, like the following that applies to Saddam Hussain:"kills millions, brutally supresses all opposition and all human rights, hires the worst profesional torturers and rapists in history"

You know I assumed that George Senior was full of shit when he called Saddam "another Hitler".

I was wrong. The problem here is that our media doesn't care enough to actually inform us of all the slaughter and oppression around the world and our local do-gooder activists are so busy hating their republican neighbors that they couldn't be bothered to check out the possibilty that they are occasionally right.

Cognitive dissonance makes it easier to believe whatever propaganda is floating around as long as it isn't our propaganda.

The situation in the Middle east is complicated, so of course we know nothing about it. It's scary but the people currently in the White House actually know more about that issue than the activists.

I don't want the "total information awareness" geeks reading my email. But you know, I can oppose some policies of my government without doing a full "you evil bastard" hissy fit.

I'm sorry, I don't mean to ask the stupidest question ever, but how does gravity have speed? The last I was taught on the subject (and believe me, it was a while ago) was that gravity was a force, but didn't have mass. Doesn't something need to have mass in order to have speed?

You're confusion arises because you were taught elementary Newtonian physics. In general relativity, one learns that any "information" cannot travel faster than light. Gravity is considered information because if you feel a gravitational force on you, you know that there is a body out there acting on you. That is, you have information about it (you could even estimate its mass by measuring the tug it exerts on you).

In Newtonian physics, lots of things are assumed to happen instantaneously (like gravity) so they don't have a speed per se. But in general relativity, everything has a speed -- and that speed is no greater than the speed of light.

"In general relativity, one learns that any "information" cannot travel faster than light"

What about quantum pairs? Move them apart, and a change in one is reflected intantly in the other.

That's why I specifically said "In general relativity...". Quantum pairs are from the theory of quantum mechanics, not general relativity. Physicists have been working hard to try to combine relativity and quantum into a single unified theory. However, problems arise when the two theories predict different things -- such as the quantum pairs example you listed. According to relativity, there would be a finite time lag for the change to be reflected in the second entity of the pair whereas quantum would say that the change is instantaneous.

Incidently, I heard that a few years ago an experiment was performed on quantum pairs and, sure enough, the change was indeed instantaneous. Can anyone else corroborate this?

Something doesnt have to have mass to have speed. Plonk a stone on a pond, you create waves. The waves have speed but dont have "mass" per se. Gravity in general relativity is similar. The entire universe is immersed in Space-time (kinda like water in a pond). Objects inside spacetime tend to "warp" spacetime. Like poking a finger into a stretched rubber sheet, creating a depression. So objects near them tend to tumble into the depression - bingo! this is gravity. This is what general relativity says. Now if this is true, warping in spacetime cannot occur instantaneously (no wave travels faster than light) as assumed in newtonian mechanics. This is what has been proved now. "Distortions in space-time does not propagate faster than light" or in other words "gravity does not travel faster than light". So no "mass" or "moving object" is involved per se.

...a topic like this to be a bit more precise in the summary. There's a signifigant difference between.95 times the speed of light, and the speed of light. Not to mention the large.25 margin of error. Which theoretically shouldn't be able to get to +.25 anyhow.

It turns out you can derive e = mc^2 with very little effort. All it takes is a spot of calculus and a bit of relativity. All you need to know is that Work = delta KE and that mass is really m0 / (1-v^2/c^2)^.5, something previous physicists had derived.

Since work is the integral of dMomentum / dt with respect to dx, you can mux that around to get, eventually, Ke = (the mass formula)*c^2 - m0 * c^2. What einstein said that was amazingly brilliant is that the equation fits Total E = KE + PE. He called that bit with the mass formula Total E, so therefore m0*c^2 = PE!! HA!, you are a smart as Einstein now!! (just kidding)

"One of the problems has to do with the speed of light and the difficulties involved in trying to exceed it. You can't. Nothing travels faster than the speed of light with the possible exception of bad news, which obeys its own special laws. The Hingefreel people of Arkintoofle Minor did try to build spaceships that were powered by bad news but they didn't work particularly well and were so extremely unwelcome whenever they arrived anywhere that there wasn't really any point in being there."

"Kopeikin found another way. He reworked the equations of general relativity to express the gravitational field of a moving body in terms of its mass, velocity and the speed of gravity. If you could measure the gravitational field of Jupiter, while knowing its mass and velocity, you could work out the speed of gravity."

The theory of relativity was appearantly used to detect the speed of gravity. This would be fine if the theory of relativity didn't assume a speed of gravity. Basically, all he did was prove his given. So, if eggs are green, then eggs are green!

The theory of relativity was appearantly used to detect the speed of gravity. This would be fine if the theory of relativity didn't assume a speed of gravity. Basically, all he did was prove his given. So, if eggs are green, then eggs are green!

sigh

You can't prove a physical theory - you can either show that it fits experimental evidence (in which case it might be right), or that it doesn't (in which case you've disproved it).

This experiment shows that a key assumption of GR is consistent with real life. That's it. That's all we can do, and that's all that is being claimed - observations of Jupiter give (roughly) the results we'd expect if gravity travels at c.

Unless you have a plausible alternative hypothesis, experiments that agree with your hypothesis tell you essentially nothing.

This observation is meaningful only in hindsight. An experiment like this one has the potential to disconfirm the hypothesis as well. The fact that it did not do so is significant, albeit not so significant as the alternative.

The slur against physicists is unjustified, particularly the "elevate it to dogma" line. If you want to test a hypothesis, you first assume that it is correct, then try to prove the assumption wrong. If you have a better method, please share it.

The sun couldn't suddenly disappear, although that scenario works for the purpose of explaining the speed of gravity. Consider this alternative.

Take the sun and instantly accellerate it to almost the speed of light, toward a collision course with Earth. For most of the 8 minutes between acceleration and collision, nobody would notice anything, as light, all other energy, and gravity would all present the sun as occupying its original location.

However, brief moments before the collision, the sun's change of accelleration toward earth will be noticed. Of course, you're noticing the change that happened 93 million miles away, even though the sun is about to impact. However, one second later, the sun will appear to be almost 186000 miles closer, and it will FEEL like it's 186000 miles closer. Suddenly the gravitational accelleration has increased to reflect the new position of the sun. But within that second, you get all the accumulated influences of gravity over a much larger stretch of space than just the 186000 miles it travelled in that time. Since the sun is moving at almost the speed of light, let's say 99% of it, after 99 seconds, the influence of the sun's gravity will only be 1 second ahead of the sun. However, within that one second between the position of the sun and the gravitational influence of the sun is contained the gravitational influence of the sun over the last 99 seconds. You get the combined force in 1 second that you normally would have gotten in 99. So when the Sun's influence is finally felt by Earth, you will not get a force that implies a steady rise in gravitational force of a sun massed object until impact, you'll get a very quick rise in force of an object that is, generally, about 99 times as large as the sun.

And if you remember relativity, when an object is travelling near the speed of light, the mass increases. So the theory at least makes sense. Here's another thing to ponder. If an object the size of the sun suddenly acquired the 99x its mass, would it not either collapse upon itself, or expand rapidly, nova, and the core would collapse upon itself, causing the same result, a singularity, with a small event horizon. And it will be this singularity that will collide with Earth, ripping through it in a fraction of a second, and the sudden, combined gravitational effect on earth will cause it to very suddenly pull out of it's orbit toward the origninal center of gravity of the sun, with a nice city sized hole carved through it.

Ok, this had no purpose at all, but it was interesting to think about. Go on with your business... nothing to see here. Rant over.

If an object the size of the sun suddenly acquired the 99x its mass, would it not either collapse upon itself, or expand rapidly, nova, and the core would collapse upon itself, causing the same result, a singularity, with a small event horizon.

the slightly longer answer is "because in the sun's inertial reference frame (i am going to leave gen.rel out of this) the sun still has the same mass."

if you don't understand what I just said, read more about special relativity, kay?

And if you remember relativity, when an object is travelling near the speed of light, the mass increases. So the theory at least makes sense. Here's another thing to ponder. If an object the size of the sun suddenly acquired the 99x its mass, would it not either collapse upon itself, or expand rapidly, nova, and the core would collapse upon itself, causing the same result, a singularity, with a small event horizon. And it will be this singularity that will collide with Earth, ripping through it in a fraction of a second, and the sudden, combined gravitational effect on earth will cause it to very suddenly pull out of it's orbit toward the origninal center of gravity of the sun, with a nice city sized hole carved through it.

point of note: a "nova" is what happens when fresh yummy hydrogen falls on a white dwarf. Boom! A "supernova" is what you were talking about. Confusing the two is a little dangerous, because they're two completely different processes.

Depends on the mechanism of acceleration, really. If it's merely "moving" at a Lorentz factor of 100, then no, of course not, because all you did was Lorentz boost the system, which you can always do. In the Sun's rest frame, it's fine still, of course. In the boosted frame, it's also incredibly flattened (like a pancake - by a factor of 100, no less) but amazingly enough, you can still work out hydrostatic equilibrium for it, and determine that yes, it is still in equilibrium, and not going to blow up. Beauty of relativity - laws of physics are Lorentz boost invariant.

However, if you're actually accelerating the thing, now that's a different story. You (still) won't make it go supernova, because you're NOT actually increasing the number of particles inside it, and that's what breaks hydrostatic equilibrium - pressure generated versus gravity, and BOTH of those change in the boosted frame - but you WILL screw it up really badly by sending pressure bubbles through the whole thing. Since the Sun isn't a rigid body, you'll probably strip the chromosphere right off of it, and leave the core bare. This, however, won't due much except really really confuse distant astronomers.

While this is very interesting, is the speed of the propogation of gravity constant or can it be affected by certain conditions? This brings to mind the experiments at slowing down light in a special supercooled gel (is this an Einstein-Bose condensate?).

I don't think I like the idea of light being the fastest anything can travel, though. Perhaps it is for many things, but what happens if some forces travel at speeds faster (or multiples), or perhaps simple fractions, and we discount those readings instead of seeing if the old model can be adapted or remade? Well, many questions, few answers from me.

Does anyone remember the 'gravity shielding' story a while back, where a spinning superconductor was supposedly responsible for changes in weight? Podkletnov comes up in a google search for 'superconductor gravity shield' but I haven't heard anything further about it.

Also, what about magnetic forces? How do those work, and at what speed do they 'travel' ?

So, really, they're triumphantly announcing that the speed of the light is somewhere between 0.7 c and 1.2 c, and just supposing it has to be c for everything to make sense.

Physicists have been accused of being loose with rigour, but this is really stretching it.

That's an excellent measurement for astrophysics. Recall, there was a recent announcement that astronomers are 95% certain that the age of the universe is between 11 and 20 billion (thousand million in the UK) years old. That's 15.5 plus or minus 29%.

If you read the original paper [arxiv.org] proposing the measurements back in July, the technique requires interferometric measurements timed to within picoseconds (1e-12 seconds) to give an accuracy of at best plus or minus 10%. That translates to pegging the apparent position of a little speck of light (and radio waves) in the sky to within five millionths of a second of arc. (Roughly speaking, that's the apparent width of a bacterium at twenty miles.) I think that they did a pretty good job to be able to call the number to within 25%, especially given that nobody has ever attempted this sort of measurement before.

No doubt it will be refined in the future; meanwhile, it's another piece of evidence which supports a subtle result general relativity. GR is a really neat theory, in that it made predictions and had consequences that we are still only beginning to be able to test nearly a century later. Even more interesting, it has yet to be contradicted by a reproducible experimental result. Hats off to Einstein, yet again.

...you see, it's all component forces. If you look at the free body diagram of my car travelling on the road, you'll see the normal force, force of gravity, and my velocity in the x direction. As I mentioned, one of these component forces is gravity, labelled FsubG. It was recently discovered, and posted on slashdot, that the speed of said force is 3x10^7m/s.

And THAT'S why, officer, your radar reported that I was going 60 in a 40 zone!

It was recently discovered, and posted on slashdot, that the speed of said force is 3x10^7m/s.

And THAT'S why, officer, your radar reported that I was going 60 in a 40 zone!

Well, no wonder you had a problem. The speed of light is ten times slower in your universe that in that of the officer's. When his radar beam slowed passing into your frame of reference, your apparent speed increased proportionally.

I asked my physics teacher the very same exact question- "if the sun disappeared, would the earth fly off from its orbit instantly, or would it take about 8 minutes?" He goes "it would be like snipping a cord- instantaneous". Discouraged, I went into the slashdot-posting, linux compiling netadmin that I am today, never knowing the true path of lab coats, leather gauntlets, and welding glasses that is physics- How dare you Stockwell! You stole my life with an assumption and I want my five years back!!!

Ole Roemer measured the speed of light back in 1676 by measuring the time difference between predicted and observed eclipses of Io by Jupiter. It's amazing that Jupiter was once again utilized to provide the first measurement of the speed of gravity.

Wondering how on earth the can explain +.25 from.95c when, according to general relativity, nothing goes faster than c? Listen up. Although I wasn't privy to how they performed this particular experiement, I've participated in other studies in the past, and have a good handle on how they are performed. What follows is my understanding of how they obtained the results that they did.

To perform the experiment, numerous (probably several thousand) measurements are taken, but due to imprecision in the process of taking the measurements (imperfect measuring equipment, human error, etc) you get a variety of results. These answers could vary from well below c to well above it. If Einstein was right and nothing propogates faster than c, the higher results could only be attributed to imprecise measuremements, but you can't throw those measurements out if you are trying to be objective.

At the end of the process, you have something vaguely resembling a normal bell curve, where the height of the curve at a point along the x axis (velocity) is a measurement of the relative frequency with which that speed of gravity was obtained as a measurement. The total area under the curve will be exactly 1. In many cases, the curve may not be symmetric, but for an experiment such as this, you are unlikely to obtain an assymetric curve (Central limit theorem of statistics, or some such thing). A line right down the middle of the curve shows the measured average result (.95c).

A confidence interval is then picked (it is a shame that this interval is not mentioned in the article, but it is almost assuredly at least 95%, probably even 99%, or 99.9%). This percentage is converted to decimal (95%=.95, 99%=.99, etc), and a symmetric region around the average score with that area is blocked off. This blocked off area has a minimum X component of.7125c, and a maximum X component of 1.1875c, the difference between each of these and the average measured velocity being.2375c, which is 25% of.95c.

And that's where the 25% margin of error comes from -- for their desired level of confidence, the variance in measured results was off by no more than 25% of the value that was actually obtained as the mean.

Since the value of 'c' lies WELL within the bounds of the margin of error of the experiment, and pre-existing theories support the speed of gravity being c, this experiment supports those theories. It is important to note that this experiment did not prove anything, it only failed to disprove that the speed of gravity is anything other than something very close to c.

Strange, 20 years ago I was taught other people had experimental evidence agreeing with a prediction that the effects of gravity move at light speed:

In 1882 Simon Newcomb observed an excessive perturbation in precession of the orbit of mercury, to the tune of 43 seconds of arc per century. In 1915, Albert Einstein showed this could be explained by the propogation of gravitic wave effects at the speed of light...

Actually, most (537 arc seconds per century) of the precession is explainable by Newtonian mechanics as perturbations caused by the other planets, Einstein did in fact use propogation velocity of gravity as 300,000,000 m/s to predict an extra precession of about 45 arc seconds per century. The "classical" component is the limit of gravitic influence propogating at infinite velocity.

Some mention of Newcomb's observations and observed/expected precession here [saao.ac.za]

If Gravity's speed is equal to that of the speed of Light, then how do you explain the pull descrepencies between blackholes and low graivty environments? Go to the moon, you'll notice that the gravitational pull there is much lower than that of Earth's. And Earth's is far far less then a blackhole's gravitational pull.

So how can one say that Gravity's pull is as fast as the speed of Light when Gravity itself doesn't stay constant in different environments? I never heard light not traveling the "speed of light" so it's a bit confusing.

Ao, from what I gather, blackholes have so much gravitational pull that even light can't escape. Which suggests to me that Gravtiy is stronger than light. It would also suggest to me that gravity is is faster than light because of this. I don't have any sources to back this up, all of this is just my train of thought in words here.

I'd appreciate a simple-as-possible answer as to why my train of thinking is wrong, as i said, i'm no scientist, but this topic is interesting none the less:)

This paper [ldolphin.org] gives a good case for gravity traveling faster than light and I'm pretty sure all the working Newtonian gravity calculations assume instantaneous gravity:

"Standard experimental techniques exist to determine the propagation speed of forces. When we apply these techniques to gravity, they all yield propagation speeds too great to measure, substantially faster than lightspeed. This is because gravity, in contrast to light, has no detectable aberration or propagation delay for its action, even for cases (such as binary pulsars) where sources of gravity accelerate significantly during the light time from source to target"

What if gravity has different properties from a long way away, such as intergalactic distances?

I've often wondered lately if perhaps gravity is both a repulsive and an attractive force. For local (i.e. interstellar) distances, the attractive force prevails. But for really vast (intergalactic) distances, it might act as a repulsive force. This could partly explain why the galaxies are accelerating away from each other.

Physicists don't have much of an idea what dark energy is... maybe it's just gravity, and Newton's law needs an amendment.

I've never heard this idea proposed, but it would make a certain kind of sense to me if it turned out to be the case.

And when you don't know what you're talking about it's easy to just mouth off. Did you read the paper to see what's going on? Maybe there are some details in the experiment you'd actually like to criticize constructively?

"If you could measure the gravitational
field of Jupiter, while knowing its mass
and velocity, you could work out the speed
of gravity."

Now, let's consider those three variables.

The first, we measured by its effect on radio
waves from the occlusion of a pulsar. Check.

The third only matters relative to the inertial
frame of the Earth, so we can actually measure
that as well. Check.

The second, however, provides a bit more of a
problem, and I have pointed this out in the
past with "proof" of traditional physical
concepts using astronomical objects and
events. Basically, our idea of Jupiter's
mass results from how it interacts with other
large gravitating objects (for example, Kepler's
third applied to the period of and distance to
one of its satellites).

Applying that same "fact" back into an
equasion designed to verify something about
gravity commits the classical flaw of logic
known as "circular reasoning". Thus,

I think we should assume more of the author. Think of it this way:
Using General Relativity, you can predict what the gravitational field will be. His experiment measured what the field actually was. If the predictions match the measurement, the theory is confirmed (or at least not disproven).

Yes, the speed of magnetism. The particle which mediates electromagnetic interactions is the photon which propagates at the speed of light. So if a magnet is suddenly given a push in one direction then there is a delay before distant particles notice a change in the field of that magnet.

This is an analogous result for gravity and the postulated graviton particles.

It's one thing to not understand something, we all have our fields of expertise. But assuming you know everything based on some limited high schooling makes you the saddest kind of idiot.

No, gravity WAVES travel at the speed of light. Gravity waves are changes in gravity, such as the increase you'd feel if a large mass suddenly moved towards you. Even if you were travelling away from something at the speed of light, the gravity "field" would already be ahead of you.

It's like if you're in a boat moving faster than a wave that's chasing you, you may avoid the wave but the water is still there.

Well... almost. Because electrons have mass, they have inertia... so they resist being kicked around, and as electrons on one side get close producing sufficient electrostatic repulsion to kick them further towards the other side, there ends up being a slight delay before the equalibrium is reachieved. Because the individual electrostatic fields of the electrons themselves propogate at exactly c, the actual rate at which the entire phenomena propogates ends up being somewhat slower, in general no more than.95c. If the electrons didn't have mass, the speed of electricity flow would definitely be exactly c.

Anywways, that's more or less how my high school science teacher explained it.

Gravitomagnetic effects make everything work out consistently. Read this FAQ [ucr.edu]. No conservation laws or symmetries are violated. (By the way, some mass-energy is radiated as gravitational waves, and the objects do spin faster and spiral into each other, but this is an extremely weak process, visible only in closely orbiting neutron stars.)